In this paper, I would like to discuss two main points in Martin Heidegger’s philosophy . First , what does Heidegger ( 1884 – 1976 ) mean by ‘’ authentic existence ‘’ ? Second , what does the term Mit-sein mean in his thought ?Heidegger’s philosophy has been the subject of many critical discussions, but Heidegger says that his philosophical ideas cannot be calssified as existence philosophy . He claims that he is not concerned man’s existence but he is interested in Being in its totality . Thus Heidegger’s view is that philosophy must return to the truth of Being because only through a rediscovery of Being that can modern man be protected from the chaos which threatens him . Also ,when discussing the question of Being . Heidegger points out that Being is indefinable . It is , in fact , a general concept and yet it is not well understood. The generality of the problem involves the exclusion of the possibility to answer the question , what is the meaning of Being ? But in asking this question , this means that the answer in present in our mind ans Being in present to us.